Evaluating the feasibility of large displacement naturally aspirated 750 RPM double-roller axial powerplant engines. Axial engines are also called "barrell engines".

Naturally aspirated, CR=16.5 for producer-gas (appr 50..55 Nm / liter of displacement equivalent to 4300 Watt / liter on shaft, 4kWe on generator).


Gensets usually achieve 50-110 kWe / ton of weight (the more usual configuration is 1500 RPM engines with traditional crankshaft and connection-rods).

Even worse (and nonsense) > 2000 EUR / liter (NA engines from Russia, India and China sometimes approach 1800 EUR / liter). Generator is only +40 .. 70 EUR / kWe (even with Leroy-Sommer).

An engine with the same-displacement as 6 double-pistons (d=8.5 inch) is about 200-300k EUR. Easy to tell that something in the market is not right. Nice playground for the enterprenous.


A more reasonable engine would have 6 double-pistons. The engine draft has only 4, and one removed for visibility.

The "camplate" or sinusoidal cam engine can be lower RPM and higher torque. This seems a disadvantage in electricity generator (1500 RPM generators are cheapest, lower RPM generators cost more).

In any case, the method of

can improve efficiency for obvious thermodynamical reasons (like the regenerator in a Stirling engine).

Piston Rings - the most delicate parts

[Hastings] offers 8.5 inch (=215.9mm) piston ring-sets. Affordable (40-50 USD/ring or $200/ringset in 2011) and available (~always in stock) also in small quantities.

They have 5/32, 8/32 and 10/32 " width. Sets for 4-5 grooves.

It is unclear if Hastings offers oversized rings. The usual increment is 0.01" (0.25 mm).

Chinese (or locally made) liners are OK, but the highest quality pistonrings should be available for them to avoid risks of poor quality rings.

Cylinder liners - displacement

For 750 RPM engines, 340 mm bore (and >400 mm stroke) is very common, and reasonable (Caterpillar, Wartsila). Anyway, max 215.9 mm sounds better for a "smaller", still reasonable size engine.

2 * 6 cylinders (154 liter total, appr 640 kWe). 2*4 (103 liter) would be another option.

Wet cyl liners or dry liners ?

ideas ?

Double roller

Advantages compared to traditional connecting rods:

horizontal twin-piston (align their pair) with double roller "swashplate" design.


Again: appears vertical on this pic, but would be horizontal in actuality

Big rollers are 110..160 kN radial force depending on bore. (bearing for this is appr 150 mm OD, and 3kg)

Side rollers counteract the resulting side forces, and relieve the piston rings from the extreme stresses that wear conn-rod engines in 20000 hours.

Lots of sensors desired to help installation and operation, and detect anomalies early on eg.

With today's automation these are trivial (related solutions seems to have been patented almost every decade in the 20th century, often hard to see anything non-trivial in these patents, or anything new at the time of filing).


Valves, valve-seat

Preferrably compatible with an engine family (Caterpillar, Cummins, whatever...).

intake valves

Exhaust valve:

Caterpillar valves

The availability of valve guides and seals must be considered also. (and possibly whole individual cyl heads if available).


1 camplate at each end of the engine


Mechanical or hydraulic ?

But see [Nissan lifter guts]. The internal diameter that counts (where the oil pressure acts during valve lifting) is much smaller even for the nissan, around 12mm.


Rockers are probably the best choice for a camplate (instead of camshaft) engine.

Nice rocker arrangements:


Oil pump

Coolant pump

Starter motor

The 3.4MW V20 GE Jenbacher engine (turbo-natural gas 1500 RPM is 21656 Nm, assuming 180 Nm /liter the total displacement can be around 120 liter) is started by 2 x 9 kW electric motor (24V each, ~400A current). It can be started by just 1 of the starters operating. A similar starter would seem suitable for the slightly bigger displacement barrell engine.

A [24V 7.8 kW caterpillar marine starter] 27.6 kg, $195

It is theoretically possible to use the syncronous generator as a starter

However, this (with the high current relays and added complexity) might be more costly (even with 400V but most likely with a 6-10kV generator) than a dedicated starter motor.

Embedded Steam Engine - doubling as starter

A 2nd axial engine on same shaft, or just 3 steam cylinders (arranged eg as double expansion 1+2 cyl) would allow the engine to self-start on steam or compressed air ("decompressor") like giant ship engines do. During normal operation the steam cylinders would provide added power and efficiency using exhaust heat to produce steam. This would mean +2 cam track on one side (on top of the 2 or 4 required for the ICE cylinders). Steam-oil contamination could be a problem. Also, the steam regulations are very strict. Likely only feasible later, possibly with larger engines.

[swashplate steam engine]